When I try to run SQL Server 2008 R2 Express I get error 26 indicating that the Server could not be found. The system worked fine last week and was not used for a week and when I tried it I get the error.

I installed Service Pack 1 and it worked fine, the first time used it,an hour later it produced the same error.

I installed SP2 but still get the error and I don't know where to turn from here.